Automatic vehicular transmissions are often provided with a planetary gear train which, includes one or more planetary gear sets, each of which may utilize braking bands and other torque control devices frictionally to engage and release selected members of each planetary gear set in order to obtain the desired function of the planetary gear train. The operator selects the drive range, neutral, forward or reverse, and the transmission automatically changes gear ratios in relation to the vehicle speed and the engine torque input, as permitted within the selected drive range.
A planetary gear set consists of a center or sun gear, an internal gear and a planetary carrier assembly which includes and supports the smaller planet gears or pinions. When the sun gear is held stationary and power is applied to the internal gear, the planetary gears rotate in response to the power applied to the internal gear and thus "walk" circumferentially about the fixed sun gear to effect rotation of the carrier assembly in the same direction as the direction in which the internal gear is being rotated.
When any two members of the planetary gear set rotate in the same direction and at the same speed, the third member is forced to turn at the same speed. For example, when the sun gear and the internal gear rotate in the same direction and at the same speed, the planet gears do not rotate about their own axes but rather act as wedges to lock that planetary gear set together so that it rotates as a unit.
Whenever the carrier assembly is restrained from spinning freely, and power is applied to either the sun gear or the internal gear, the planet gears act as idlers. In that way, the driven member is rotated in the opposite direction as the drive member. When, for example, the reverse drive range is selected, a band assembly may be actuated frictionally to engage the carrier assembly and restrain it against rotation so that torque applied to the sun gear will turn the internal gear in the opposite direction in order to reverse the rotational direction of the drive wheels, and thereby, reverse the direction of the vehicle itself. The present invention relates to servo mechanisms of the type employed to engage and release typical friction band assemblies.
Such a servo mechanism generally incorporates a piston assembly. The piston assembly is normally operated by the introduction of pressurized hydraulic fluid which displaces a piston to protract a servo pin the predetermined distance necessary operatively to effect engagement of the friction band assembly with the selected member of a planetary gear set. As the hydraulic fluid exits the piston assembly, the servo pin retracts to its original position and thereby allows the friction band assembly to release the selected member of the planetary gear set. The length of the servo pin is therefore selected so that it will operate the friction band assembly selectively to engage or release a rotating element in a planetary gear set in response to that axial displacement of the servo pin effected by the piston assembly.
In the configuration heretofore described, an actuating piston is included in the piston assembly, and the actuating piston axially reciprocates between its release position and its apply position. The distance between its release position and its apply position constitutes the throw or axial displacement of the actuating piston. The fixed connection between the actuating piston and the servo pin is such that when the actuating piston has reached its release position, the servo pin will be in its retracted position -- which allows the friction band assembly to release the member of the planetary gear set upon which it operates. Conversely, when the actuating piston has reached its apply position the servo pin will be in its protracted position -- which causes the friction band assembly to tighten onto the member of the planetary gear set upon which the friction band assembly operates. As the friction band assembly thus engages the selected member of the planetary gear set, rotation of that member is thereby precluded.
In many installations the length of the servo pin is selected for operation in a system wherein the relative position of all elements is fixed, but in some installations, a mechanical adjustment is available at the connection between the friction band assembly and the transmission casing from which the friction band assembly is supported. The mechanical adjustment of the friction band assembly constitutes a convenient means by which to establish optimum operation of the friction band assembly within the range of axial displacement available to the servo pin by reciprocation of the actuating piston. Such adjustments are highly desirable during assembly of the transmission, and -- because of the normal wear incident to the friction band assembly -- it has heretofore been required that periodic, manual adjustments be made during the operational life of the transmission. Unless some periodic adjustments are made, the quality of the shift effected by the transmission degrades. Even though periodic adjustments assure satisfactory operation of the transmission throughout its operational life, some owners simply procrastinate in attending to such maintenance and others don't wish to invest either the time or money required for even such routine maintenance.